1. Agmon, S.: Spectral properties of Schrödinger operators and scattering theory. Ann. Scuola Norm. Sup. Pisa U. Sci. Series4, 2 151?218 (1975)
2. Aronzajn, N.: A unique continuation theorem for solutions of elliptic partial differential equations or inequalities of second order. J. Math. Pures Appl.36, 235?250 (1957)
3. Atiyah, M.F., Patodi, V.K., Singer, I.M.: Spectral asymmetry and Riemannian geometry. I. Math. Proc. Cambridge Philos. Soc.77, 43?69 (1975)
4. Lecture Notes in Mathematics;M. Berger,1971
5. Birman, M.S.: Existence conditions for wave operators. Izv. Akad. Nauk SSSR Ser. Mat.27, 883?906 (1973) [Russ.] English Translation: Amer. Math. Soc. Transl., Series II,54 (1966) 91?118